1. Field of the Invention
The invention relates to the field of electronically scanning antennas and in particular, to millimeter-wave antennas controlled by ferrite magnetizing.
2. Description of the Prior Art
Antennas with ferrite control of a beam based on a controlled lenses technology are known in the prior art. One of the examples of such antennas is disclosed by U.S. Pat. No. 4,588,994. This antenna having small radiating aperture dimensions, not exceeding five wavelengths (5.lambda.), is not capable of providing a narrow beam. An array of several lenses contains unilluminated areas in the aperture causing high order diffraction of maximal radiation. The lenses have a complicated and large control circuit as well as large longitudinal dimensions.
The other approach for the electrical scanning on the basis of a ferrite control technology is disclosed by U.S. Pat. No. 4,785,304. This patent provides an array of waveguide-slot antennas of traveling waves, with each waveguide formed as a solid ferrite rod having a rectangular cross-section containing a metallized surface. Radiating slots are disposed at the top region of the rod. When the ferrite is magnetized in the longitudinal direction, scanning is carried out by means of variations of the phase velocity of the operating waveguide mode. The dimensions and weight of this antenna are substantially reduced compared to those of other prior art antennas. Nevertheless, this antenna contains multiple drawbacks. In this respect, when the beam is normal to the antenna, an in-phase adding of reflections (so called &lt;&lt;normality effect&gt;&gt;) occurs causing the gain drop and pattern diagram distortion. If the ferrite is demagnetized, then while the beam is at the center of the scanning sector, another mode of the same direction is intensively excited. This diminishes gain and produces greater side lobe levels. Furthermore, in the antenna of this patent, since the magnetic circuit is not closed, additional phase distortions appear. This occurs due to non-homogeneous magnetization of the ferrite rod along its length. The shortened circuit formed by the metallization around the rod, among other reasons, substantially increases the time of beam switching and the control power consumption.
A similar antenna is disclosed by the Russian Patent No. 2,000,633, in which each waveguide is formed by two ferrite layers. A thin dielectric element made of a material having substantial dielectric permeability is placed between the ferrite layers. Since only a bottom surface is metallized, the waveguide of this antenna is of an open type. Radiating elements are in the form of microstrip dipoles situated at the top surface. The waveguide operates at only a single low order mode also representing an operating mode. In this antenna, the high order modes have significantly different phase velocities and therefore are poorly excited. Due to the waveguide non-reciprocity of the waveguide, the &lt;&lt;normality effect&gt;&gt; can be avoided in the entire scanning sector including the beam area situated along the normal to the antenna. Since the magnetic circuit is of the closed type, resembling a toroid, the power consumption is decreased. The antenna has a low profile design and low weight characteristics.
An important drawback of the above discussed prior art antenna is that all waveguides have to be substantially identical and thus, should be equally magnetized during the scanning. This leads to the excessive tolerance requirements substantially raising the price of the antenna. Furthermore, it is quite difficult to provide homogeneous magnetizing of the ferrite layers even under perfect conditions. This is because, the magnetizing is maximal in the central region, and diminishes in the area of outer rows of dipoles. Consequently, upon magnetizing of the ferrite layers, i.e. while the antenna beam deviates from an average position, the characteristics of this prior art antenna deteriorate.
Another prior art antenna is disclosed by IEEE International Symposium on Phase Array System and Technology Publication (Boston, Mass. 1996). This antenna is formed with three layer ferrite-dielectric structure and contains only one row of radiating dipoles providing a narrow beam in H-plane (containing the vectors of the magnetic field and passing along the axis of the waveguide). The directivity in the E-plane (containing the vectors of the electrical field and directed transversely the axis of the waveguide) is achieved by two additional metal elements directly disposed at the top surface of a top ferrite layer at both sides of an array of radiating dipoles. The inner-walls of these metal elements facing each other are positioned at an angle to a vertical plane and form diverging walls resembling a horn structure.
In this antenna, the metal elements forming the horn structure are directly connected to the top ferrite layer significantly affecting the properties of the ferrite dielectric waveguide. This generates parasitic modes including those propagating transversely with respect to the waveguide axis. Upon reaching outer edges of the three-layer waveguide, such parasitic modes radiate a part of the power energy from the side surfaces of the antenna which results in substantial deterioration of the efficiency of the antenna. This prior art antenna is typically unsuitable for independent usage and is intended to be utilized as a line scanning irradiator for a parabolic cylindrical antenna.